1. Field of the Invention
The present invention relates to a gas generator for an air bag, and to an air bag system.
2. Description of the Related Art
An air bag system mounted on various vehicles such as automobiles has an object that, when the vehicle collides at a high speed, an air bag (bag body), which is rapidly inflated by gas, supports a vehicle occupant so as to prevent the occupant from being injured in colliding with hard portions of a vehicle interior such as a steering wheel and a windshield glass due to inertia of the occupant.
In general, such an air bag system comprises a gas generator which is operated by a collision of a vehicle to discharge gas, and an air bag to which the discharged gas is introduced to be inflated. Conventionally, as such a gas generator, there has been provided a gas generator having a structure that gas generating means is, burnt to generate combustion gas, is accommodated in the gas generator, and gas generated by the combustion of the gas generating means is discharged from the gas generator.
In the gas generator for an air bag which generates combustion gas by a combustion of the gas generating means, an operating time of the gas generator is an extremely short, that is, about 60 milliseconds. However, since a combustion gas generated by the combustion of the gas generating means has high temperature, the following case is considered after the gas generator has been operated; more specifically, a temperature of an outer container (housing) of the gas generator gradually increases due to conduction of heat generated by combustion of the gas generating means. In particular, in the case of a gas generator including coolant means in its outer container so that high temperature gas is not directly introduced into an air bag, the coolant means cools combustion gas and combustion residues by heat exchange. For this reason, the coolant means itself becomes hot, and then, heat of the coolant means is transferred to the outer container; as a result, the temperature of the outer container increases. The aforesaid gas generator is installed in a module case together with an air bag, and is mounted on various vehicles as an air bag system. In this case, considering safety after the gas generator has been operated, in the gas generator, it is desirable to prevent a temperature increase on a side where the air bag is inflated, that is, on the diffuser shell side, as much as possible. This is because the following problem is considered. An air bag quickly deflates after inflating due to a collision. When an occupant accidentally touches the diffuser shell, in the case where the surface temperature on the diffuser shell side is too high, the occupant may burn himself due to the increased temperature of the diffuser shell side.
However, there has not been provided a gas generator for an air bag, which effectively reduces or inhibits a temperature increase in an outer shell container on a diffuser side after the gas generator has been operated (in particular, a temperature increase by conduction of heat generated by combustion of the gas generating means).
Incidentally, in the past, International Publication No. W096/10495 discloses a gas generator having a structure in which a heat insulating member is provided above and/or below an outer chamber including a gas generating agent. In the aforesaid gas generator, the heat insulating member is disposed between the gas generating agent and the container (housing) in order not to affect a burning velocity of the gas generating agent by ambient temperature. In this case, the gas generator has an effect of preventing a temperature increase of a housing, where the heat insulating member is provided, after the gas generator is operated. However, the surface temperature under the heat insulating member is substantially high, and the heat insulating member peels from the outer container by gas jetting at a high speed. As a result, there is the case where no achievement of the purpose is made.
In view of the above problem in the prior art, it is therefore an object of the present invention to provide a gas generator for an air bag, which can restrict conduction of heat generated by combustion of gas generating means after the generator is operated to effectively reduce or inhibit a temperature increase of a diffuser side in an outer shell container, and thus, can reduce the maximum surface temperature of the diffuser.
According to the present invention, the diffuser is prevented from becoming too hot and the air bag can be used with safety.
To achieve the above object, the present invention provides a gas generator for an air bag comprising a housing which has a diffuser shell with at least one gas discharge port and a closure shell for forming an accommodating space together with the diffuser shell, and gas generating means, installed in the housing, to be ignited and burnt by igniting means and to generate a combustion gas, wherein the gas generator further includes heat transfer restricting means for making a conduction of heat generated by combustion of the gas generating means to the diffuser shell side less than that to the closure shell side. In the case that the coolant means for cooling the combustion gas generated by the combustion of the gas generating means is accommodated in the housing, it is desirable that the heat transfer restricting means reduces or inhibits heat conduction from the coolant means to the diffuser shell. In general, the coolant means is used so as not to directly introduce the combustion gas generated by the combustion of the gas generating means into the air bag. The conduction of heat generated by combustion of the gas generating means to the diffuser shell after the gas generator has been operated is mainly made from the coolant means. For this reason, the heat transfer restricting means is disposed between the coolant means and the diffuser shell, and thereby, it is possible to effectively reduce or inhibit heat conduction from the coolant means.
The gas generator of the present invention includes the heat transfer restricting means, and thereby, it is possible to make the maximum surface temperature of the diffuser shell (i.e., the highest temperature of a surface temperature range of the diffuser shell after the gas generating agent has been burnt) lower than that of the closure shell. The aforesaid heat transfer restricting means includes, for instance, a heat transfer suppressing member disposed between the diffuse shell and a member such as the coolant means for conducting a heat generated by the combustion of the gas generating means to the diffuser shell, or a space formed between both components. The heat transfer suppressing member reduces or inhibits conduction of heat generated by combustion of the gas generating means from the coolant means and the like to the diffuser shell, and must function so that at least heat generated by combustion of the gas generating means is not conducted to the diffuser shell. The aforesaid heat transfer suppressing member includes, for instance, a heat insulating member formed by using a proper heat insulating material, a space forming member for forming a heat insulating space or the like. Desirably, the heat insulating member is sufficiently durable to a combustion temperature of the gas generating means and exhibits a heat insulating effect. Preferably, a ceramic or ceramic fiber is used as the heat insulating member, in particular. Moreover, a space formed by the space forming member disposed between the coolant means and the diffuser shell serves to reduce or inhibit a heat conduction from the coolant means to the diffuser shell. Therefore, preferably, the space is a heat insulating space at least capable of reducing or inhibiting a heat conduction between the coolant means and the diffuser shell. In particular, the space forming member preferably has a structure where the heat insulating space can be formed. In addition, in the case of using the space formed between the coolant means and the diffuser shell as the heat transfer restricting means, the coolant means and the diffuser shell do not directly make contact with each other, and no member of conducting heat is disposed between both components. Thus, a heat of the coolant means is not directly conducted to the diffuser shell, and is conducted thereto by only its radiation and/or convection. As a result, it is possible to restrict an increase of the surface temperature of the diffuser shell.
As described above, the heat transfer suppressing member is disposed between the diffuser shell and a member such as the coolant means which absorbs heat generated by combustion of the gas generating agent. In the case of interposing the heat transfer suppressing member between the coolant means and an inner surface of a ceiling portion of the diffuser shell, in order to prevent a short-pass, in which a combustion gas passes between the heat transfer suppressing member and the coolant means, and/or between the heat transfer suppressing member and the diffuser shell, a short-pass preventing member, which covers an area ranging from the upper inner surface of the coolant means to an inner surface of the ceiling portion of the diffuser shell, may be formed integrally with the heat transfer suppressing member. For example, in the case of using a heat insulating member as the heat transfer suppressing member, the heat insulating member and the short-pass preventing member may be integrally formed. Also, in the case of using a space forming member as the heat transfer restricting means, for example, the space forming member and the short-pass preventing member may be integrally formed as shown in an Embodiment 1 of FIG. 1 which will be described later.
In addition to the coolant means in the conventional case which comprises a metallic wire mesh for cooling combustion gas generated by combustion of the gas generating means, the coolant means may also include the filter means as described above which is generally used for removing residues in combustion gas and also performs a cooling function of the combustion gas. Moreover, a coolant/filter, which has a complicated spacing structure with the use of a multi-layer wire mesh body so as to cool combustion gas and collects combustion residues in the combustion gas, may also be used. In general, each of these coolant means is formed into a substantially cylindrical shape, and is arranged outside a portion where the gas generating means is positioned. An outer periphery of the coolant means may be provided with an outer layer which comprises a multi-layer wire mesh body, a multi-perforated cylinder, an annular belt or the like in order to prevent the coolant means from being bulged by the combustion of the gas generating means.
In the gas generator of the present invention, a conventional azide based gas generating material(s) on the basis of inorganic azide, for example, natrium azide (sodium azide) can be used as the gas generating means which is contained in the housing and is burnt to generate a combustion gas, and besides, a non-azide based gas generating material(s), which is not based on the inorganic azide can also be used. However, in general, the non-azide based gas generating material is higher in combustion temperature than the azide based gas generating material, and the former has a higher calorific value permit gas generating quantity than the latter. Further, in the gas generator of the present invention, since the heat transfer restricting means can make the maximum surface temperature of the diffuser shell lower than that of the closure shell, a further remarkable effect can be obtained by using the non-azide based gas generating material as the gas generating means. Various materials have been proposed as the non-azide based gas generating compositions. For example, the following materials are known as the agent; more specifically, an organic compound containing nitrogen, such as tetrazole, triazole, or metal salts of these, etc., and an oxidizing agent containing oxygen, such as alkali metal nitrate, etc., and compositions using as their fuel and nitrogen source, triaminoguanidine nitrate, carbohydrazide, nitroguanizine, etc., and using as their oxidizing agent the nitrate, chlorate, perchlorate, etc., of an alkali metal or alkali earth metal. These non-azide based gas generating materials are applicable to the present invention; however, the gas generating agent of the present invention is properly selected on demand of a burning velocity, non-toxicity and combustion temperature, without being limited to these. The gas generating agent is used in a proper shape such as a pellet, a wafer, a hollow cylinder, a disk, a single or multiple perforated body, etc.
The housing is formed by casting, forging or pressing the diffuser shell having the gas discharge port and the closure shell for forming an accommodating space together with the diffuser shell, and by joining both shells together. Joining of both shells is carried out by various welding methods, for example, electron beam welding, laser welding, TIG arc welding, projection welding or the like. In the case where the diffuser shell and the closure shell are formed by pressing various steel plate and sheet such as stainless copper plate, it is possible to readily manufacture both shells, and to achieve a reduction in its manufacture cost. Moreover, these shells are formed into a cylinder, that is, a simple shape, and thereby, its pressing is easy. Preferably, a stainless steel plate is used as the material of the diffuser shell and the closure shell, and a copper plate with nickel plating may be used. Incidentally, an interior space of the housing is divided into two chambers by providing the inner cylindrical member inside the housing, and thereafter, various members may be properly contained therein.
Further, the housing is usually provided therein with igniting means which is operated when an impact is detected, and ignites and burns the gas generating means. The igniting means may be either of a mechanical ignition type igniting means which is operated upon sensing an impact exclusively by means of a mechanical device or mechanism, and an electric ignition type igniting means which is operated by an electrical signal transmitted from an impact sensor for sensing an impact. Preferably, the electric ignition type igniting means is used as the igniting means. The electric ignition type igniting means comprises: an electrical sensor which senses an impact exclusively by an electrical mechanism; an igniter which is operated by an electrical signal transmitted from the sensor upon sensing an impact; and a transfer charge ignited by the operation of the igniter and then burns. As the electrical sensor, there is, for example, a semiconductor type acceleration sensor or the like.
The aforesaid gas generator for an air bag is accommodated in a module case together with an air bag (bag body) to which gas generated by the gas generator is introduced to expand the bag, and thus, an air bag system is constructed. In the air bag system, the gas generator is operated at the point of time when the impact sensor senses an impact, and combustion gas is discharged from the gas discharge port of the housing. Then, the combustion gas flows into the air bag, and thereby, the air bag breaks the module cover and inflates, and thus, forms a cushion for absorbing an impact between hard structures in a vehicle and a vehicle occupant.
Further, to achieve the above object, the present invention provides a method for reducing temperature increase of a diffuser shell in a gas generator for an air bag comprising a housing which has a diffuser shell with at least one gas discharge port and a closure shell for forming an accommodating space together with the diffuser shell and gas generating means, installed in the housing, to be ignited and burnt by igniting means and to generate a combustion gas, wherein a conduction of heat generated by combustion of the gas generating means to the diffuser shell is reduced or inhibited to reduce a temperature increase of an outer surface of the diffuser shell by combustion of the gas generating means.
The conduction of combustion gas of the gas generating means to the diffuser shell is reduced or inhibited by interposing a space forming member and a heat transfer suppressing member such as a heat insulating member between the diffuser shell and a coolant means. The coolant means may be a coolant/filter which is formed to have a complicated spacing structure with the use of the multi-layer wire mesh body.
The present invention provides a gas generator for an air bag, which can effectively reduce or inhibit a temperature increase on a diffuser side by heat generated by combustion of gas generating means after the gas generator has been operated, and thus, can reduce the maximum surface temperature of the diffuser.